Accelerometer and method for making same

ABSTRACT

An accelerometer based on monocrystalline silicon. A moving mass is connected to a fixed frame by suspension arms. The sensor is produced in the stack of two silicon wafers in which stop and counter-stop functions are produced so as to limit the amplitude of the movements of the moving mass.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention is that of deformable microstructures withtwo-directional operation, particularly that of accelerometerscomprising a moving mass connected to a fixed frame via slendersuspension arms.

2. Discussion of the Background

The accelerations experienced by the moving mass may typically bedetected thanks to the presence of piezoresistive gauges on the slendersuspension arms. Such piezoresistive gauges may also control anelectromagnetic device for controlling the moving mass, in order tocompensate for its displacements.

In this case, the accelerometer strain gauges are intended to generate,by means of an electronic circuit, a supply current for a coil which,using phenomena of electromagnetic induction, compensates for adisplacement of the moving mass under the effect of external conditions,for example seismic movements.

At the present time, such accelerometers can be built using siliconwafers, using silicon etching techniques developed in the context of themanufacture of semiconductor electronic components. The use of suchtechnologies allows the moving mass and the elements associated with itto be produced by a collective process on slices of silicon and thusallows a series of acceleration sensors to be defined, using a limitednumber of technological steps.

Typically, the manufacture of this type of sensor comprises assemblingseveral silicon wafers, at least one central wafer in which the movingmass is produced, and two outer wafers having stop elements for themoving mass, thus making it possible to limit the amplitude of themovements which the said moving mass can be made to experience, and thusto optimize the protection of the moving mass and of the suspensionarms. An example of an accelerometer made up of at least three siliconwafers according to the known art is illustrated in FIG. 1.

More specifically, FIG. 1 illustrates a cross-section through this typeof accelerometer the manufacture of which is, in particular, the resultof assembling 4 silicon wafers, a so-called upper wafer constitutingpart of the fixed frame and comprising upper stops Bs, two centralwafers constituting the moving mass 1 and part of the fixed frame and afourth wafer, called the lower wafer, comprising part of the fixed frameand a stop plane Bi for the lower face of the moving mass.

To simplify the architecture of this type of accelerometer and make themethod of manufacturing it easier, accelerometers are known which resultfrom assembling two wafers in which all the elements and functions aredefined (moving mass, suspension arms, frame) as disclosed inApplication WO 95/04284 or U.S. Pat. No. 5,121, 633.

SUMMARY OF THE INVENTION

The application also proposes a simplified architecture in which thestops and counter-stops functions are cunningly produced.

More specifically, the subject of the invention is an accelerometercomprising a moving mass connected to a fixed frame, the moving mass andthe fixed frame being defined in a stack of two monocrystalline siliconwafers known as the upper wafer and the lower wafer, characterized inthat:

the upper wafer comprising [sic]:

the upper part of the moving mass, including a first and a secondcounter-stop along an axis Y defined in the plane of the moving mass;

the upper part of the fixed frame, including a first and a second stopalong an axis X perpendicular to the axis Y and defined in the plane ofthe moving mass;

the upper part of the moving mass and the upper part of the fixed framebeing connected by suspension arms;

the lower wafer comprising [sic]:

the lower part of the moving mass, including

a third and a fourth counter-stop along the axis X;

the lower part of the fixed frame, including a third and a fourth stopalong the axis Y;

the first and second counter-stops being opposite the third and fourthstops respectively;

the third and fourth counter-stops being opposite the first and secondstops respectively.

By convention, the stops are defined as fixed elements of theaccelerometer, against which counter-stops, which are moving elements ofthe accelerometer, come to rest.

According to an alternative form of the invention, the accelerometer ischaracterized in that:

the upper part of the moving mass comprises a first central part andfirst and second immobilizing arms lying one on either side of the saidfirst central part, along the axis Y;

the upper part of the fixed frame comprises a peripheral part and firstand second arms lying inside the peripheral part, along the axis X, theupper wafer comprising first and second suspension arms connecting thecentral part of the upper part of the moving mass to the first andsecond arms of the peripheral part;

the lower part of the moving mass comprises a central part and third andfourth immobilizing arms lying one on either side of the said secondcentral part, along the axis X;

the lower part of the fixed frame comprises a peripheral part and thirdand fourth arms lying inside the peripheral part along the axis Y;

the first and second counter-stops being defined in the first and secondimmobilizing arms;

the first and second stops being defined in the first and second arms;

the third and fourth counter-stops being defined in the third and fourthimmobilizing arms;

the third and fourth stops being defined in the third and fourth arms.

According to an alternative form of the invention, the wafers 6 and 7 ofthe accelerometer comprise slender guide arms connecting the moving mass1 to the frame 3 in such a way that only movements of the moving massalong an axis Z perpendicular to the plane of the said mass arepossible. Advantageously, these arms may lie along two axes oriented at45° to the axes X and Y and lying towards the top of the wafer 6 andtowards the bottom of the lower wafer 7, that is to say in planes whichcorrespond to the outer plane of the wafer 6 and to the outer plane ofthe wafer 7, the inner planes of the said wafers corresponding to theplane of contact of these said wafers.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood, and other advantages willbecome apparent from reading the description which will follow, which isgiven without implied limitation and with support from the appendedfigures, among which:

FIG. 1 illustrates an accelerometer according to the prior art;

FIG. 2a illustrates the upper wafer used in one example of anaccelerometer according to the invention;

FIG. 2b illustrates the lower wafer used in one example of anaccelerometer according to the invention;

FIG. 3a illustrates a cross-section on a plane depicted by the axis AA'of the assembly of wafers 6 and 7 depicted in FIGS. 2a and 2b;

FIG. 3b illustrates a cross-section on a plane depicted by the axis BB'of the assembly of wafers 6 and 7 depicted in FIGS. 2a and 2b;

FIG. 4a illustrates one example of a wafer 6 with four guide arms;

FIG. 4b illustrates one example of a wafer 7 with four guide arms;

FIGS. 5a-5h illustrates the steps in a method for producing a lowerwafer used in an accelerometer according to the invention;

FIGS. 6a-6e illustrates the steps in the method of producing an upperwafer used in an accelerometer according to the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

One embodiment of the accelerometer according to the invention isillustrated by FIGS. 2 and 3 together. FIGS. 2a and 2b relate to thewafers 6 and 7 the stacking of which allows the moving mass 1 connectedto a fixed frame 3 to be defined.

FIG. 2a relates to the upper wafer 6. This wafer 6 comprises the upperpart 11 of the moving mass consisting of a central part 13 and of twoimmobilizing arms 14 and 15 lying one on either side of the central partand, in FIG. 2a, oriented along the axis Y.

The upper wafer 6 also comprises the upper part 31 of the fixed frame,consisting of a peripheral part 33 and of two arms 34 and 35, which inFIG. 2a are oriented along the axis X.

Part 42 of the immobilizing arm 15, and part 41 of the immobilizing arm14 constitute counter-stops on the part 11 of the moving mass.

Part 43 of the arm 34 and part 44 of the arm 35 constitute stops on thepart 31 of the fixed frame.

This wafer 6 also comprises two suspension arms 21 and 22 connecting thecentral part 13 of the moving mass to the arms 34 and 35 secured to thefixed frame.

FIG. 2b relates to the lower wafer 7. This wafer 7 comprises the lowerpart 12 of the moving mass consisting of a central part 16 and of twoimmobilizing arms 17 and 18 which in the figure are oriented along theaxis X.

The wafer 7 also comprises the lower part 32 of the fixed frame,consisting of a peripheral part 36 and. of two arms 37 and 38 which inthe figure are oriented along the axis Y.

Part 42 of the immobilizing arm 15 and part 41 of the immobilizing arm14 constitute counter-stops facing the stop 48 part of the arm 38 andthe stop 47 part of the arm 37, respectively.

Likewise, part 45 of the immobilizing arm 17 and part 46 of theimmobilizing arm 18 constitute counter-stops facing the stop 43 of thearm 44 and the stop 44 of the arm 45, respectively.

FIGS. 3a and 3b illustrate how the various stops and the variouscounter-stops can be produced.

The arms 17, 18, 37 and 38 of the lower wafer 7 have the samearchitecture. They consist of a second element of thickness e₁ less thanthe thickness e₀ of a silicon wafer. The difference in thickness betweenthese arms and the other parts of the silicon wafer makes it possible toform stops or counter-stops for the counter-stops or stops whichrespectively face them.

The arms 14, 15, 34 and 35 of the upper wafer 6 all have the samearchitecture. They consist of a single element of thickness e₀, part ofeach arm facing the thinned part which constitutes an arm in the lowerwafer 7.

We are going to describe one example of the method for producing anaccelerometer according to the invention, built from a stack of twosilicon wafers.

FIGS. 5 and 6 respectively illustrate the steps in the method forproducing the lower wafer, without the piezoresistive gauge, and themethod for producing the upper wafer with piezoresistive gauges.

More precisely, FIG. 5 as a whole describes the production of theelements 36, 17, 16 depicted in FIG. 3a, and FIG. 6 as a whole describesthe production of the elements 33, 43, 21, 13 depicted in FIG. 3a.

To give the accelerometer a certain degree of robustness, the upper andlower wafers may comprise slender guide arms, which hold the moving massin place relative to the fixed frame.

FIGS. 4a and 4b illustrate one example of a configuration in which theupper wafer comprises four guide arms 81, 82, 83, 84 (FIG. 4a) and thelower wafer also comprises four guide arms 85, 86, 87, 88 (FIG. 4b).Typically, the guide arms may have a thickness similar to that of thesuspension arms 21 and 22 and be situated, on the one hand, in the upperpart of the wafer 6 (in the same plane as the suspension arms) and, onthe other hand, be situated in the lower part of the wafer 7.

In order to construct the various elements of the lower wafer 7, use maybe made of a substrate of the SIMOX type which corresponds to amonocrystalline silicon substrate in which a layer of oxide 90 isembedded (FIG. 5a).

Two layers of oxide or of nitride 91 and 92 are then produced on the twofaces of the substrate of the SIMOX type (FIG. 5b).

A mask is produced by photolithography and etching at the layer 91 (FIG.5c).

This mask is refined by a second etching step, as illustrated in FIG.5d.

The silicon is chemically etched in the conventional way in the regionswhich have no protective layer (FIG. 5e).

The layer of oxide or of nitride 91 is then thinned down as illustratedin FIG. 5f.

A second step of etching of the silicon is then carried out in order todefine the counter-stop 45 in the immobilizing arm 17 (FIG. 5g).

Finally, a laser etching step is carried out, which leads to theimmobilizing arm 17 being separated from the peripheral part 36 of thefixed frame (FIG. 5h).

In order to achieve the desired functions on the upper water 6, it maybe particularly advantageous to use a silicon wafer in which twoinsulating layers 93 and 94 have been made, the said wafer moreoverbeing covered with an insulating layer 95 on the opposite face.

The two embedded layers 93 and 94 may be produced in various ways.

Use may be made of a wafer of the SIMOX type, in which the layer 93 isembedded in the conventional way. In the conventional way, a layer 4000Å thick may be embedded at a depth of about 4000 Å. It is then possibleto grow the layer of monocrystalline silicon by epitaxy. The layer ofoxide 94 can then be embedded in this epitaxially grown layer (FIG. 6a).

Another method consists in the high-temperature welding of two siliconwafers one of which is oxidized at its surface. This yields an assemblyof two silicon wafers joined together at the oxide layer which thenconstitutes the layer 93. One of the wafers can then be machined so asto define the desired thickness of silicon in which oxygen can beembedded ionically to define the layer 94.

The thin layer 96 makes it possible to define silicon elements which areisolated from the rest of the wafers and may constitute piezoelectricgauges which are better defined and calibrated than those which resultfrom implantation within a silicon substrate in a conventional way (FIG.6b).

The piezoelectric gauges are protected by a layer 97 of nitride, forexample, or of oxide, or of both and openings are produced locally atthe layer 97 to allow for the electrical connections (FIG. 6c).

A conductive layer can then be deposited and then etched in order, inthe known way, to form tracks for connecting each gauge, these not beingdepicted in FIG. 6c.

A mask is produced by etching through the layer 95 as illustrated inFIG. 5d.

The manufacture of this mask is followed by a step of etching thesilicon chemically, making it possible to define the suspension arm 21,the arm 34 with its stop 43, and the upper central part of the movingmass 13.

The two silicon wafers 6 and 7 thus machined may be welded by heatingtypically to a temperature of the order of 1000° C.

The two wafers may be joined together before the laser cuttingillustrated in FIG. 5h is performed, thus making it possible to detachthe immobilzing arm 17 from the peripheral part of the frame 36, so asto make-the said assembly easier.

All the steps described hereinabove are also used to produce the arms14, 15, 37 and 38 along the axis Y.

The slender guide arms 81, 82, 83, 84 can also be produced in the upperwafer in the same way as the slender suspension arms and in the sameplane, thanks to the etching arresting layer.

The slender guide arms 85, 86, 87, 88 can also be produced in the lowerwafer thanks to the layer of silicon which lies between the layers ofoxide 90 and 92.

What is claimed is:
 1. An accelerometer comprising:a moving massconnected to a fixed frame, wherein the moving mass and the fixed frameare defined in a stack of upper and lower monocrystalline siliconwafers; said upper monocrystalline silicon wafer including:an upper partof the moving mass, including two upper counter-stops along a first axisdefined in a plane of the moving mass and positioned on opposite sidesof a central portion of the upper part of the moving mass; an upper partof the fixed frame, including two upper stops along a second axisperpendicular to the first axis and defined in the plane of the movingmass and positioned on opposite sides of the central portion of theupper part of the moving mass; wherein the upper part of the moving massand the upper part of the fixed frame are connected by first and secondsuspension arms; said lower monocrystalline silicon wafer including:alower part of the moving mass, including two lower counter-stops alongthe second axis positioned on opposite sides of a central portion of thelower part of the moving mass; a lower part of the fixed frame,including two lower stops along the first axis positioned on oppositesides of the central portion of the lower part of the moving mass;wherein the two upper counter-stops are opposite the two lower stops;and wherein the two lower counter-stops are opposite the two upperstops.
 2. An accelerometer comprising:a moving mass connected to a fixedframe wherein the moving mass and the fixed frame are defined in a stackof upper and lower monocrystalline silicon wafers; said uppermonocrystalline silicon wafer including:an upper part of the moving massincluding two upper counter-stops along a first axis defined in a planeof the moving mass; an upper part of the fixed frame, including twoupper stops along a second axis perpendicular to the first axis anddefined in the plane of the moving mass; wherein the upper part of themoving mass and the upper part of the fixed frame are connected by firstand second suspension arms; said lower monocrystalline silicon waferincluding:a lower part of the moving mass, including two lowercounter-stops along the second axis; a lower part of the fixed frame,including two lower stops along the first axis; wherein the two uppercounter-stops are opposite the two lower stops; wherein the two lowercounter-stops are opposite the two upper stops; and wherein: in saidupper monocrystalline silicon wafer,the upper part of the moving massincludes an upper central part and two upper immobilizing arms onopposite sides of said upper central part along the first axis; and theupper part of the fixed frame includes an upper peripheral part and twoupper frame arms inside the upper peripheral part along the second axis,and said upper monocrystalline wafer further including two suspensionarms connecting the upper central part of the upper part of the movingmass to the two upper frame arms of the upper peripheral part; in saidlower monocrystalline silicon wafer,the lower part of the moving massincludes a lower central part and two lower immobilizing arms onopposite sides of the lower central part, along the second axis; and thelower part of the fixed frame includes a lower peripheral part and twolower frame arms inside the lower peripheral part, along the secondaxis; the two upper counter-stops are defined in the two upperimmobilizing arms; the two upper stops are defined in the two upperframe arms; the two lower counter-stops are defined in the two lowerimmobilizing arms; the two lower stops are defined in the two lowerframe arms.
 3. An accelerometer according to claim 2, wherein the upperand lower monocrystalline wafers further include guide arms connectingthe moving mass to the fixed frame, said guide arms extending along twoaxes oriented at 45° to the first and second axes.
 4. An accelerometeraccording to claim 3, wherein the upper monocrystalline silicon wafercomprises four upper guide arms and the lower monocrystalline siliconwafer comprises four lower guide arms, said four upper guide armsextending towards a top of the upper monocrystalline silicon wafer, andsaid four lower guide arms extending towards a bottom of the lowermonocrystalline silicon wafer.
 5. An accelerometer according to claim 2,wherein the first and second suspension arms comprise piezoresistivegauges.
 6. An accelerometer according to claim 3, wherein the first andsecond suspension arms comprise piezoresistive gauges.
 7. Anaccelerometer according to claim 4, wherein the first and secondsuspension arms comprise piezoresistive gauges.
 8. An accelerometeraccording to claim 1, further comprising a system forelectromagnetically controlling the moving mass.
 9. An accelerometeraccording to claim 2, further comprising a system forelectromagnetically controlling the moving mass.
 10. An accelerometeraccording to claim 3, further comprising a system forelectromagnetically controlling the moving mass.
 11. An accelerometeraccording to claim 4, further comprising a system forelectromagnetically controlling the moving mass.
 12. An accelerometeraccording to claim 5, further comprising a system forelectromagnetically controlling the moving mass.
 13. An accelerometeraccording to claim 6, further comprising a system forelectromagnetically controlling the moving mass.
 14. An accelerometeraccording to claim 7, further comprising a system forelectromagnetically controlling the moving mass.